Patent GB-981804-A (GB)
Title: Apparatus for recording signals upon or reproducing signals from magnetic tape
Inventor: BALDWIN JOHN LEWIS EDWIN
IPC: G11B15/14
Filed: 1962-07-24
Abstract: 981,804. Television. RANK-BUSH MURPHY Ltd. July 24, 1963 [July 24, 1962], No. 28432/62. Heading H4F. The invention relates to magnetic recording and reproducing apparatus for frequencymodulated television signals of the type in which the signals occupy discontinuous tracks extending transversely of a magnetic tape and is concerned with monitoring the signal processing circuits by the periodic injection into the signal channel of an auxiliary signal of predetermined frequency. For signal recording, Fig. 1, the original television signal at terminal 7 is applied through a clamp and amplifier 8 to frequency modulate an oscillation in a modulator 9 and the resulting signal is then applied through an amplifier 10, record/reproduce switch 6 and slip rings 5 to the transducer heads carried on drum 3. In carrying out the invention the signal from amplifier 10 is also applied through a demodulator 12 to an oscilloscope 13, the circuit incorporating a pair of gates 11 and 15 controlled by a transistor switching circuit 16, 17, whereby the signal may be periodically interrupted and replaced by a standard signal of predetermined frequency from a generator 20. Circuit 16, 17 is controlled by a pulse generator 19 which responds to each horizontal synchronizing pulse included in the signal at the output demodulator 12 and functions to cause the signal from generator 19 to be substituted for the recording signal for an interval occurring wholly within the synchronizing pulse interval. Generator 19 also controls standard frequency generator 20 so that the signal may have alternately frequencies corresponding to peak white and the black level or the level of the tips of the synchronizing signals. For signal reproduction, Fig. 3, the signals from the transducer heads 33a-33d are applied through slip rings 35 and amplifiers 40-43 to gate stages 44-47 which are controlled in sequence as each head engages tape 31 so as to form a continuous signal at output terminal 48. In accordance with the invention a fifth gate 50 is provided which is supplied with a signal of predetermined frequency from a generator 49, the gate also being connected to output terminal 48 and being operated in timed relationship with gates 44-47 so that at each change-over from one gate to another the opened gate is briefly closed whilst gate 50 is opened to inject the signal from generator 49. Control of the gates is effected by transistor switching circuits 51-54 which are operated by signals derived from the rotation of drum 33 which carries the transducer heads and horizontal synchronizing signals contained in the reproduced signal. The operation is such that the change-over from one gate to another occurs during the " front porch " preceding the first horizontal synchronizing pulse following the tape engagement by a transducer head, whilst the standard signal from generator 49 is injected wholly within the period of that horizontal synchronizing pulse. The signal may have alternately frequencies corresponding to peak white and the black level or the level of the tips of the synchronizing signals. Alternatively the signal may have frequencies corresponding to all three levels in a sequence at horizontal synchronizing frequency or the frequency at which gates 44-47 are switched. A detailed transistor circuit for gates 44-47 and 50 and switching circuits 51-54 is described with reference to Fig. 6 (not shown). Provision may be made for preventing the injection of the standard frequency signal during at least a part of the vertical blanking interval, more particularly during the equalizing pulses and the vertical synchronizing pulse.